Relax netlock to shared netlock and push it down to mrt_sysctl_mfc()

[Vitaliy Makkoveev]

mrt_rtwalk_mfcsysctl() performs read-only access to protected data, so
rtable_walk() could be called with shared netlock.

Index: sys/netinet/ip_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_input.c,v
retrieving revision 1.384
diff -u -p -r1.384 ip_input.c
--- sys/netinet/ip_input.c      16 May 2023 19:36:00 -0000      1.384
+++ sys/netinet/ip_input.c      17 May 2023 09:59:16 -0000
@@ -1712,10 +1712,7 @@ ip_sysctl(int *name, u_int namelen, void
        case IPCTL_MRTMFC:
                if (newp)
                        return (EPERM);
-               NET_LOCK();
-               error = mrt_sysctl_mfc(oldp, oldlenp);
-               NET_UNLOCK();
-               return (error);
+               return (mrt_sysctl_mfc(oldp, oldlenp));
        case IPCTL_MRTVIF:
                if (newp)
                        return (EPERM);
Index: sys/netinet/ip_mroute.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_mroute.c,v
retrieving revision 1.138
diff -u -p -r1.138 ip_mroute.c
--- sys/netinet/ip_mroute.c     19 Apr 2023 20:03:51 -0000      1.138
+++ sys/netinet/ip_mroute.c     17 May 2023 09:59:16 -0000
@@ -479,10 +479,12 @@ mrt_sysctl_mfc(void *oldp, size_t *oldle
        msa.msa_len = *oldlenp;
        msa.msa_needed = 0;
 
+       NET_LOCK_SHARED();
        for (rtableid = 0; rtableid <= RT_TABLEID_MAX; rtableid++) {
                rtable_walk(rtableid, AF_INET, NULL, mrt_rtwalk_mfcsysctl,
                    &msa);
        }
+       NET_UNLOCK_SHARED();
 
        if (msa.msa_minfos != NULL && msa.msa_needed > 0 &&
            (error = copyout(msa.msa_minfos, oldp, msa.msa_needed)) != 0) {